Mystery current drain

[bigherb]

Keep it simple all you need is an spdt changeover switch. wire it in reverse the outputs become the inputs from the leisure battery and ign, the input becomes the output to the radio.

[MagicWagon]

Thanks bigherb. You are right, doing what you suggest would have been a lot simpler, and that's what I did in my last van.
In my current van I wired it so power would always come from the leisure battery. This power can either come via a manual switch, or via a relay, when the ignition is live,
(see the relevant section of my wiring diagram here).

My thinking was it was better to always power it from the LB rather than the main, especially when the engine is not running.
In hindsight, with my newly acquired knowledge from this thread, that is pretty pointless as it's drawing a lot more than that from elsewhere when the ignition is on.
I suppose I'm no worse off though, so I don't intend to rewire it unless I get a good opportunity or reason.
Thanks to all for your input.